Varicella zoster virus (VZV) is the cause of chickenpox (varicella) and shingles (zoster). Varicella is a primary infection in which VZV infects a naïve host. Zoster is the result of the reactivation of VZV, which has remained latent in its host, often for many years. A major paradox has impeded research on VZV for many years: VZV is highly infectious and spreads readily from an infected host to susceptible individuals, yet, it is extremely difficult to propagate in vitro, because it spreads only by direct cell-to-cell contact when it is grown in tissue culture. The dissemination of VZV among a population of susceptible subjects is mediated by cell-free virions, which are thought to be airborne. Cell-to-cell spread of VZV in vitro does not depend on cell-free virions, which are not released in viable form by infected cells in tissue culture. Instead, cells that are infected in vitro fuse with their uninfected neighbors enabling infection to be transferred intracellularly.